Over the past three decades there has been increasing interest in the use of flexible intermediate bulk containers (hereinafter bulk bags) for handling flowable materials such as chemicals, minerals, fertilizers, foodstuffs, grains and other agricultural products, etc. The advantages resulting from the use of bulk bags include low weight, reduced cost, versatility and, in the case of reusable receptacles, low return freight costs.
Fabrics are often utilized in the construction of various types of bulk containers where strength, flexibility and durability are important. Originally, such containers were fabricated from natural fibers. More recently, however, synthetic fibers manufactured from polypropylene, polyethylene or other polymeric materials have come into almost exclusive use. The popularity of synthetic fibers can be attributed to the fact that they are generally stronger and more durable than their natural fiber counterparts.
Even with the advances in fabric construction resulting from the shift from natural to synthetic fibers, fabrics in general possess qualities that render their use undesirable in certain applications. For example, the friction that occurs as dry flowable materials are handled by fabric receptacles tends to cause a significant build-up and retention of static electric charge within the receptacle. Discharge of the generated static electric build-up is often difficult, if not impossible, to control because fabrics are generally not electrically conductive materials. However, controlled discharge is imperative as static electric potential poses a significant danger of fire or explosion resulting from a static generated electrical spark.
In an effort to address the undesirable static electric discharge characteristic of fabrics, bag manufacturers covered one side of the fabric with a metallic foil-like layer. An adhesive was applied to affix the foil-like layer to the plastic fabric. The foil-like layer was typically comprised of aluminum or some other electrically conductive metal. The foil-covered fabric was then used to construct the receptacle, for example, with the foil side of the fabric comprising the interior surface. The foil layer provided an electrically conductive surface exposed to the flowable materials through which static electricity generated during material handling and was discharged to an appropriate ground.
While adequately discharging static electric build-up if undamaged, the foil-like layer was susceptible to abrasion, tearing and separation from the fabric layer through normal use of the receptacle. For example, in filling, transporting and/or emptying of foil-covered fabric receptacles, abrasion between the flowable material and the foil-like layer tended to cause the foil-like layer to tear and/or separate from the fabric layer. The cumulative effect of such abrasion quickly reduced the effectiveness of the foil-like layer as a static electric discharge surface. Furthermore, tearing of the foil often resulted in a release of foil particles and flakes from the fabric, thereby contaminating the contained flowable materials.
To address the problems experienced with foil-covered fabrics, U.S. Pat. No. 4,833,008, issued to Norwin C. Derby, discloses a metalized fabric comprised of a woven plastic base fabric laminated to a metalized plastic film. The plastic base fabric is preferably a woven polypropylene fabric, and the plastic film is preferably an extruded polypropylene film. The plastic film is metalized through a vapor deposition process whereby a thin film of electrically conductive material is deposited on one side of the plastic film. The woven plastic fabric and the metalized plastic film are then laminated together through use of a plastic adhesive. Unlike foil covered fabrics, the thin conductive layer deposited on the plastic film is not subject to tearing or flaking; however, it is susceptible to chemical reactions.
U.S. Pat. No. 5,244,281, issued to Norwin C. Derby discloses bags made from the fabric disclosed in the Derby '008 Patent in combination with fabrics impregnated with anti-static compounds. The bags disclosed in the Derby '281 Patent provide satisfactory anti-static capabilities. Still further developments in the art are disclosed in copending application Ser. No. 8/474,378 filed by Norwin C. Derby and Craig A. Nickell on Jun. 7, 1995 and assigned to the assignee of the present application.
A more recent development in the art of static dissipating bulk bags is the "D" type bag which dissipates static electricity without the necessity of grounding by means of corona discharge. At present, "D" type bags are manufactured using Crohmiq.RTM. fabric available from Linq Industrial Fabrics, which is in turn manufactured by weaving into the fabric Nega-stat.RTM. fibers manufactured by DuPont. The Nega-stat fibers, while conductive, do not have sufficient capacitance to produce an energetic spark discharge. Rather, because of small size and geometry, the Nega-stat fibers facilitate corona discharge. The Crohmiq.RTM. fabric may also have an extrusion coating of a polymeric material having an anti-static loading of either glycol monosterate or Techmer PM 1530-E4.
"D" type bulk bags perform satisfactorily if no liner is required. Heretofore, no liner is available for use in "D" type bags. This is limiting because many powders have particle sizes that are too small to be effectively contained by bulk bags not having liners. Also, some regulated materials require a liner for shipment. Thus, a need exists for a bulk bag liner for use with a "D" type bag, and for a bulk bag liner that functions similarly to a "D" type bag.
Another recent development in the art of static dissipating bulk bags is the "B" type bag which is characterized by a breakdown voltage of less than 4,000 volts, thereby eliminating the possibility of propagating brush discharge. A "B" type bag is also non-conductive which eliminates the possibility of a spark discharge. Thus, in the use of a "B" type bag the only possible discharges of static electricity are brush discharges and corona discharges. "B" type bags are therefore adapted for use in almost all environments not involving flammable gases or solvents, and may be used for many sensitive dusts.
"B" type bags are less costly to manufacture than "D" type bags and are therefore preferable in suitable applications. Like "D" type bags, "B" type bags perform well when no liner is required. At present, no liner is available for use with "B" type bags. Since many bulk bag applications require a liner, a need exists for a liner suitable for use with "B" type bags or having the static electricity dissipating characteristics of a "B" type bag.